The present invention generally relates to a biosensor and, more particularly, to a biosensor provided with an electrode refreshing arrangement which is arranged to apply to an electrode of the biosensor a bias voltage having opposite polarity to that at the time of measurement, before starting the measurement.
Conventionally, a biosensor has attracted attention to the characteristics thereof in that it can detect considerably complicated organic compounds, etc. with a high sensitivity and with a high selectivity, and studies have been extended to various kinds of such biosensors.
For a representative of such a biosensor as referred to above, it has been proposed to detect the presence or absence of a substance, or the amount of a substance present in an object to be measured on the basis of an electrical signal output from an electrode which is secured with physiologically active material, and applied which is supplied with a predetermined normal bias voltage; for example, it has been proposed that H.sub.2 O.sub.2 generated as a result of the reaction between an enzyme secured to an enzyme film and an object to be measured is arranged to be led to the surface of an electrode through an H.sub.2 O.sub.2 transmissive film, with a working electrode and a counter electrode made of Pt, so that an electrical signal corresponding to the amount of H.sub.2 O.sub.2 penetrating the film is output to detect the presence or absence of a substance in the object to be measured or the amount of the substance present in the object. Concretely speaking, the working electrode is given a 0.6 V normal bias voltage with respect to the standard counter electrode.
According to the above-described biosensor, when the measurement of the object is continuously carried out in the state where the working electrode is supplied with normal bias voltage, a disturbance film against the circulation of electricity such as an oxide film is disadvantageously formed on the surface of the working electrode, deteriorating the activity of the working electrode. Therefore, for solving this problem, it has also been proposed, as disclosed in the published specification of Japanese Pat. Publication (unexamined) No. 60-155959, that after several measurement operations have been conducted, a reverse bias voltage is supplied to the working electrode while the measurement is not carried out, that is, about -0.6 V is added to the working electrode in the above-described example, so as to thereby remove the disturbance film and revive the activity of the working electrode, so that the output signal is returned to its original level.
Thus, in the manner as above, the deteriorated measuring sensitivity of the electrode can be restored by the application of the predetermined reverse bias voltage to the working electrode when the measurement is not carried out and, the measurement can be performed with a high sensitivity again.
However, according to the above-described prior art, the voltage drop (referred to as an overvoltage hereinafter) in the counter electrode is changed by the flowing current since a constant voltage is applied for refreshing use between the working electrode and the counter electrode, resulting in the change of the voltage applied to the working electrode. Accordingly, it is not certain that constant refreshing effect can always be achieved, namely, the activity restoration of the working electrode can always be realized. Since the change of the refreshing effect will change the activity condition of the working electrode after the refreshing thereof, the level of the output electrode signal is unfavorably changed.
In addition, while the refreshing operation is being done, a current which is approximately ten times the current during the time of measurement flows, which necessitates a larger area for the counter electrode. Therefore, the arrangement as a whole is inevitably bulky, with an increase in its manufacturing cost.
Referring to FIG. 7, there is shown a circuit diagram of a prior art bias supplying arrangement in a biosensor which is provided with a reference electrode 26 in addition to a working electrode 24 and a counter electrode 27. In the bias supplying arrangement of FIG. 7, a direct current source 21 is connected for measurement use between a non-inverting input terminal of an operational amplifier 23 and ground, and moreover, a resistance 22 for current-voltage conversion use is connected between an inverting input terminal and an output terminal of the operational amplifier 23. The working electrode 24 is connected to the inverting input terminal of the operational amplifier 23. An operational amplifier 25 which has its non-inverting input terminal connected to ground has its inverting input terminal connected to the reference electrode 26 and its output terminal connected to the counter electrode 27.
Employing the above construction, even when the counter electrode 27 is not so large, if the potential difference between the working electrode 24 and the reference electrode 26 is continued to be held constant, the counter electrode 27 is able to be rid of the influences of the overvoltage and thus a correct measurement can be obtained.
As shown in FIG. 5, in the construction of FIG. 7, since the level of the output signal, as shown with a chain line B, is not stable before considerable time has passed after supply of electricity, the measurement is generally started after sufficient time for stabilization of the level of the output signal has passed (approximately one hour at the longest), and moreover, it is arranged to shorten the time required before the subsequent measurement by maintaining the state where the electricity is always supplied once after the supply thereof.
However, the stabilized level of the output signal is much lower than the level of the output signal at the initial stage of the supply of electricity, and therefore the resolution is deteriorated. Moreover, if the supply of electricity is interrupted or the battery is cut off halfway through a measurement, it takes a long time before the level of the output signal is returned to be stable after the supply of electricity resumes.